Flexural strength is one of the most important properties of cardboard. In particular, cardboard used for packing requires strength and rigidity, which ensures that the package withstands the journey all the way to the consumer. Much is required from the cardboard to guarantee a high strength and rigidity, which runs counter to the aim of reducing the amount of raw materials.
Sufficient rigidity has previously been achieved by using a cardboard quality possessing a sufficiently high grammage. Attempts have been made to produce paper and cardboard qualities by reducing the amount of fibre and by replacing the fibres, for example, with fillers, which causes problems in ensuring the strength and rigidity.
Furthermore, in order to reduce the use of raw materials, layer structures have been developed, which exploit the form of the I-beam. In this structure, on the surface and the back side of a bulky middle layer there is a more strongly bonded layer, which is denser than the middle layer. The greater the distance is between the surface layers, the stronger the effect of the I-beam structure will be. If the cardboard is bent, elongation occurs on the convex side of the cardboard and, correspondingly, compression on the concave side. An opposing force is generated against the elongation and the compression, the strength of which force is affected by the thickness, the elasticity and the density of the layers.
Previously, a bulky middle layer has been produced by using mechanical pulps that have undergone a low level of beating.
A problem has arisen in that the more bulky/porous the middle layer is made, the fewer are the bonds between the fibres in it and the less is its internal strength. Also, a lower level of beating decreases the generation of bonds between the fibres, because the specific surface is reduced and the number of fibrils possessing setting ability is reduced, which results in reduced internal strength of the middle layer.
Weak bonding strength may result in a number of different problems in the cutting, finishing, processing and printing stages. For example, the use of sticky printing inks in offset printing results in strain in the z-direction, which may cause delamination of the cardboard, i.e. cracking in the z-plane. Similarly, weak bonding strength increases the amount of dust in the cutting and processing stages, as well as in later treatment of the products.
A bulky structure can also be achieved by using foam forming instead of water forming. Foam forming is described for example in the publications U.S. Pat. No. 5,164,045 and WO 991573.
In this case, the fibres are mostly not orientated with the machine direction, but instead their orientation varies more in the x-y plane and the z-direction of the cardboard. Thus, the bonding is also distributed in all directions and relatively a higher strength is achieved in the z-direction.
However, foam forming does not increase the number of bonds at a given degree of grinding, which makes it necessary to use an aid to increase the strength. An existing way of increasing the strength is to apply strengthening chemicals, which, however, have negative properties, such as their high cost and the potential negative effects on the chemistry of the wet-end, as well as their weak effect and low retention.